The invention relates generally to overvoltage protectors and, more particularly, to reducing and balancing the capacitance of an overvoltage protector in high frequency transmissions.
Signal-carrying transmission lines, such as telephone lines connected to computer modems for high frequency data transmissions, are subject to high voltage surges resulting from a lightning strike or AC power transient. Therefore, it is often necessary to provide telephone lines with surge protection devices to protect people and equipment connected to the transmission line from transient voltage and current surges. Surge protection devices that are shunt-connected are generally either clamping devices or crowbar devices. Under normal operating conditions, a clamping device acts as a high-impedance path to a protected transmission line. Ideally, a clamping device behaves as an open circuit. In actuality, however, a small amount of current may leak through the clamping device along the transmission line. When a transient voltage exceeds the normal operating voltage of the transmission line, the device behaves as a low-impedance path. Clamping devices automatically return to a high-impedance state when the line voltage returns to a normal operating level. A crowbar device begins to break down with a positive resistance until the device reaches a break-over voltage. Upon reaching the break-over voltage, the device xe2x80x9csnapsxe2x80x9d back to a low on-state voltage. The low on-state voltage means that the device dissipates less power, and thus, provides a higher surge current handling capability than does a clamping device. A disadvantage of a crowbar device is that the current through the device must fall below a specified holding current for the device to return to a non-conducting state.
Available technologies for parallel protection elements include gas-discharge/surge arrestors (also known as xe2x80x9cgas tubesxe2x80x9d), metal oxide varistors (MOVs), and solid-state devices. Examples of solid state protection devices include transient voltage suppressor (TVS) diodes and TVS thyristors. MOVs and TVS diodes operate as clamping devices, whereas gas tubes and TVS thyristors operate as crowbar devices. The use of gas tubes, MOVs, diodes and thyristors in surge protection has been a popular method for preventing injury to people and damage to equipment caused by an accidental overvoltage. Since telephone lines may be used for high-frequency data transmissions, an overvoltage protector utilizing thyristors, for example triacs, may interfere with the high frequency operation of telephone lines. Normally, telephone line overvoltage protectors are designed to protect against induced AC voltage and voltage spikes caused by lightning and power line switching. Thus, conventional overvoltage protectors utilizing thyristors are not optimized to reduce interference with the high frequency operation of telephone lines. In fact, the problems typically encountered with thyristors are: 1) an unacceptably high off-state capacitance; and 2) the capacitance varies significantly with DC bias voltage, causing excessive imbalance for high frequency transmissions. This high off-state capacitance and imbalance causes excessive attenuation and reflection at high frequencies, and thereby, interferes with high frequency transmissions over telephone lines.
For example, frequencies of up to about 30 megahertz are employed over telephone lines utilizing digital subscriber line (DSL) technology for data transmissions. An overvoltage protector utilizing thyristors has an off-state capacitance in high frequency transmissions from about 70 to about 200 picofarads (pfd). This fairly high off-state capacitance causes impedance mismatches on the twisted pair telephone line, which typically results in excessive signal attenuation and reflection (i.e., return loss) at high frequencies. Moreover, an overvoltage protector utilizing thyristors has unbalanced tip-to-ground and ring-to-ground capacitance characteristics in high frequency operation, since such high frequency characteristics are typically not considered in the design of the protector. In addition, the capacitance of individual thyristors will vary exponentially with DC voltages found on transmission lines, thereby increasing the need to balance the tip-to-ground and ring-to-ground capacitance between the twisted pair on the telephone line.
It is therefore desirable in an overvoltage protector on a telephone line to have a relatively low off-state capacitance as measured between the tip and ring wires as well as a substantially balanced tip-to-ground capacitance and ring-to-ground capacitance. As will be readily appreciated by those of skill in the art, reducing the tip-to-ground capacitance and/or the ring-to-ground capacitance will necessarily reduce the overall off-state capacitance of the overvoltage protector. As used herein, the term xe2x80x9coverall off-state capacitancexe2x80x9d refers to the capacitance of the overvoltage protection circuit as measured between the tip and ring wires. In view of the above-noted deficiencies, it is apparent that there exists a specific need for an apparatus and method for reducing and balancing the capacitance of an overvoltage protector in high frequency transmissions.
The present invention provides an apparatus and method for reducing and balancing the capacitance of an overvoltage protector in high frequency transmissions. More specifically, the present invention provides an overvoltage protection circuit having a reduced off-state capacitance so that the circuit may be used to protect signal-carrying transmission lines transmitting data at high frequencies, for example telephone lines utilizing digital subscriber line (DSL) technology. At the same time, the present invention also provides an overvoltage protection circuit having a substantially balanced off-state capacitance.
In a particular embodiment, the apparatus is an overvoltage protection circuit including a first overvoltage protection device and a first diode network electrically connected in series between a first electrical conductor and an electrical ground. The apparatus further includes a second overvoltage protection device and may include a second diode network electrically connected in series between a second electrical conductor and the electrical ground. The first overvoltage protection device and the second overvoltage protection device are preferably selected from the group consisting of a gas tube, an MOV, a TVS diode, and a TVS thyristor. If the first and second overvoltage protection devices are solid state overvoltage protectors, they may be integrated with the first and second diode networks onto a common semiconductor chip. The first diode network includes at least one pair of diodes electrically connected in parallel and arranged with opposing polarities so that the electrical currents flowing between the first electrical conductor and the electrical ground are bi-directional. Preferably, the first diode network includes two sets of two or more stacked diodes electrically connected in parallel and arranged with opposing polarities so that the electrical currents flowing between the first electrical conductor and the electrical ground are bi-directional. The second diode network may be configured the same or differently (e.g., for protection at a subscriber premises) than the first diode network, or may be eliminated from the circuit altogether (e.g., for protection at a telephone company central office) depending on the different capacitances of the first and the second overvoltage protection devices caused by voltage bias.
In another particular embodiment, the invention is a method for reducing and balancing the off-state capacitance of an overvoltage protection circuit. The method includes the first step of electrically connecting a first overvoltage protection device to a first diode network in series between a first electrical conductor and an electrical ground. The method may include the second step of electrically connecting a second overvoltage protection device to a second diode network in series between a second electrical conductor and the electrical ground. The first overvoltage protection device and the second overvoltage protection device are preferably selected from the group consisting of a gas tube, an MOV, a TVS diode, and a TVS thyristor. If the first and second overvoltage protection devices are solid state overvoltage protectors, they may be integrated with the first and second diode networks onto a common semiconductor chip. The first diode network includes at least one pair of diodes electrically connected in parallel and arranged with opposing polarities so that the electrical currents flowing between the first electrical conductor and the electrical ground are bi-directional. Preferably, the first diode network includes two sets of two or more stacked diodes electrically connected in parallel and arranged with opposing polarities so that the electrical currents flowing between the first electrical conductor and the electrical ground are bi-directional. The second diode network may be configured the same or differently (e.g., for protection at a subscriber premises) than the first diode network, or may be eliminated from the circuit altogether (e.g., for protection at a telephone company central office) depending on the different capacitances of the first and the second overvoltage protection devices caused by voltage bias.